(a) Field of the Invention
The present invention relates to a zoom lens system and, more particularly, to a zoom lens system with a focal length variable from 75 mm to 150 mm and large aperture ratio about F/2.8.
(b) Description of the Prior Art
There are many known zoom lens systems for photographing with focal lengths variable from about 75 mm to about 150 mm. However, most of them have aperture ratios about F/4 to F/3.5. This means that the F number when the stop is fully open is large, brightness of the lens system is low and, consequently, photographing conditions are limited. Moreover, as the brightness of the lens system is low, it is inconvenient in various points when deciding the composition of the photograph to be taken and focusing the lens system on the object by using the finder. Therefore, at present, users prefer bright photographic lenses with single focal length to zoom lenses.